1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and more particularly to an image forming apparatus capable of effectively coping with toner scattering and defective image transfer to occur when a toner image is transferred from a photoconductive element, intermediate image transfer body or similar image carrier to a sheet or recording medium.
2. Description of the Background Art
Today, various image forming systems are available with a printer or similar image forming apparatus. A direct image transfer system, for example, develops a latent image formed on a photoconductive element with toner and directly transfers the resulting toner image to a sheet. An intermediate image transfer system transfers a plurality of toner images of different colors to an intermediate image transfer body one above the other to thereby form a composite color image and then transfers the composite color image to a sheet.
When the toner image is transferred from the photoconductive element, intermediate image transfer body or similar image carrier to a sheet, it is likely that the toner image is degraded by some different causes, particularly toner scattering. Toner scattering refers to an occurrence that a toner image is not transferred to an expected position, but is scattered therearound and blurred thereby. Particularly, toner scattering lowers the sharpness of line images. More specifically, toner is transferred from the photoconductive element or the intermediate image transfer body to a sheet by an electric field formed at the inlet of a nip for image transfer, i.e., pretransfer as generally referred to. Further, when discharge occurs at, e.g., the inlet of the nip, the amount of charge (Q/M) of the toner decreases with the result that the Coulomb force, acting between toner grains, overcomes electrostatic adhesion, aggravating toner scattering. Various schemes have heretofore been proposed for reducing toner scattering.
For example, Japanese Patent Laid-Open Publication No. 9-236992 proposes to cause a sheet to contact a photoconductive element at a position upstream of an image transfer position (pre-contact), so that a toner image is protected from disturbance ascribable to discharge occurring at the inlet of a nip. In practice, however, it is extremely difficult to effect stable pre-contact at all times. More specifically, because flexibility of a sheet, for example, depends on environmental conditions and the kind of a sheet, it is likely that a toner image is rubbed when a sheet, carrying the toner image thereon, enters the nip.
As for primary image transfer from a photoconductive element to an intermediate image transfer body, Japanese Patent Laid-Open Publication No. 8-30119 teaches that toner scattering can be reduced if a bias of the same polarity as toner and a bias opposite in polarity to toner are applied at the upstream side and downstream side of a nip, respectively. This document, however, does not address to the dependence of a sheet on environmental conditions, which is a problem particular to secondary image transfer, and cannot be directly applied to secondary image transfer.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-20726 and 2000-19854.